This invention relates to an acoustic or elastic surface wave multimode filter which makes use of an acoustic surface wave or surface acoustic wave (SAW).
Such multimode filters are described, for example, in a paper submitted by H. F. Tiersten et al to the "1975 Ultrasonics Symposium Proceedings" of the IEEE, pages 293 and 294, under the title "Guided Acoustic Surface Wave Filters." According to the Tiersten et al paper, the multimode filter comprises a piezoelectric substrate and a pair of acoustic surface wave resonators on the substrate. Each resonator is called a "strip guide" in the Tiersten et al paper and comprises an interdigital transducer and a pair of reflecting arrays on both sides of the interdigital transduder. In outline, each resonator is rectangular and has a length and a width, namely, a pair of long sides and a pair of short sides. The resonators are arranged side by side with the long sides disposed parallel and with the short sides aligned. A spacing is left between two adjacent sides of the respective resonators. The multimode filter serves as a band-pass filter and may comprise three or more resonators to provide more than two poles. Such multimode filters may be connected in tandem to form a higher-order filter.
It is already known that the multimode filter has a relative band width dependent on the width of each resonator and the spacing between the resonators. The spacing must be as small as possible in order to make the multimode filter have a wide passband.
An acoustic surface wave multimode filter is revealed in Japanese patent application No. 130,140/1982 of Yuzo Nakazawa et al (Kokai No. 131,213/1984). According to the Japanese patent prepublication, each resonator may consist of an interdigital transducer alone. The above-described width and spacing are discussed in detail.
In various embodiments of the invention of Nakazawa et al, a bus bar is formed on the substrate between adjacent long sides of two resonators so as to be shared by the resonators. The bus bar is given a narrow width on order to render the spacing between resonators narrow. It is to be noted that the relative band width is restricted by the width of the bus bar. The bus bar has a sheet resistance. It has now been found by applicant that the sheet resistance results in increased insertion loss of the multimode filter and in a dull cutoff characteristic. Such defects become more serious when the frequencies are higher. This makes it difficult to use the multimode filter in the UHF band.